This invention relates to reverse thrust gas turbofan engines and, more particularly, to an outlet guide vane arrangement for use therein.
A major feature of future gas turbine engines will be their ability to change the direction of fan thrust by reversing the direction of flow through the fan. This will be accomplished through the incorporation of what is known as a variable pitch fan. However, it is becoming clear that more is involved in such a concept than the mere addition of a variable pitch fan to an existing, state-of-the-art gas turbine core engine. One of the subtle problems facing the industry is to find an efficient method of providing the core engine with a motive fluid stream, typically air, in both the forward and reverse thrust modes.
In particular, in such advanced engines it is contemplated that a variable pitch fan will be used to pressurize a large quantity of air in much the same manner as a conventional turbofan engine when the variable pitch fan is operating in its forward thrust mode. Typically, the majority of this air passes through a bypass duct surrounding a core engine to provide the greater part of the engine's forward thrust. The remaining air passes through an inner core duct to the core engine which drives the fan through turbine and shaft connection. In the reverse thrust mode, the direction of engine thrust is reversed to aid in aircraft braking, for example. In this mode, the pitch of the fan blades is set such that the air is drawn back through the bypass duct in the opposite direction (the direction of fan rotation remaining unchanged). The majority of the air is expelled out of what is normally the inlet of the engine. However, in order for the core engine to continue driving the fan, it must be continually supplied with a motive fluid source in the reverse thrust mode. Accordingly, a portion of the bypass duct flow is bled off and turned, essentially 180.degree., to enter the core engine duct.
Guide vanes are normally provided in gas turbofan engines to turn the absolute flow angle (swirl) back to the axial direction after being pressurized by the fan in order to maximize forward thrust in the bypass portion and to provide for axial flow entering the core engine. It is the presence of these necessary guide vanes which creates a problem in the reverse thrust mode.
In some contemplated reverse-thrust engines the flow is required to pass through guide vanes twice during its circuitous route to the core engine. First it must traverse backwards those guide vanes or portions of guide vanes associated with the bypass duct, and then traverse in the forward direction those guide vanes, or portion thereof, associated with the core engine duct. Since the camber of the latter guide vanes will be in the wrong direction during the reverse mode (absent variable geometry guide vanes), high pressure losses are expected with detrimental results in core engine performance. The higher the pressure ratio of the fan (the higher its aerodynamic loading) the greater the camber required on the guide vanes to straighten the flow in the forward thrust mode, and the more severe the problem in the reverse thrust mode.
In other contemplated reverse-thrust engines, the fan guide vane arrangement is such that a non-constant radial energy distribution by the fan could cause a stalling condition in the guide vanes, while still other arrangements require very high guide vane camber with attendant high losses. Concern over these matters has necessitated a search for a solution to the problem.